Fluorocarbon based fluids have found widespread use in industry in a number of applications, including as refrigerants, aerosol propellants, blowing agents, heat transfer media, and gaseous dielectrics. Because of the suspected environmental problems associated with the use of some of these fluids, including the relatively high global warming potentials, it is desirable to use fluids having the lowest possible greenhouse warming potential in addition to zero ozone depletion potential. Thus, there is considerable interest in developing environmentally friendlier materials for the applications mentioned above.
Tetrafluoropropenes, having zero ozone depletion and low global warming potential, have been identified as potentially filling this need. While the toxicity, boiling point, and other physical properties in this class of chemicals vary greatly from isomer to isomer, one tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (HFO-1234yf). HFO-1234yf has been disclosed to be an effective refrigerant, heat transfer medium, propellant, foaming agent, blowing agent, gaseous dielectric, sterilant carrier, polymerization medium, particulate removal fluid, carrier fluid, buffing abrasive agent, displacement drying agent and power cycle working fluid.
During one process of producing HFO-1234yf, 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) and 2-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb) are produced as a first and second intermediate compounds, respectively. Specifically, HFO-1234yf is made from a 3 reaction step process. The first step is the reaction of pentachloropropane or tetrachloropropene with HF to form HCFO-1233xf. The second step is the hydrofluorination of HCFO-1233xf with HF to form HCFC-244bb, and the third step is the dehydrochlorination reaction of HCFC-244bb to form HFO-1234yf. Incomplete reaction in the second step reaction will result in a stream containing both HCFC-244bb and HCFO-1233xf. Such a reaction is well known in the art and is described in U.S. Applications 20070007488, 20070197842, and 20090240090, the specifications of which are incorporated herein by reference.
One problem with this process is that HCFC-244bb and HCFO-1233xf are inseparable using conventional separation techniques known in the art because together they form a binary azeotrope or azeotrope-like composition, which is described in U.S. Patent Application 20090242832, the specification of which is incorporated herein by reference. If, after reaction, there remains a relatively large amount of unreacted HCFO-1233xf in the resulting crude organic material this would require most of, if not all of, the stream to be recycled back to the reactor for additional conversion before proceeding to the next reaction step. This would be very costly from a capital (equipment size) requirement and a manufacturing standpoint. In addition, the presence of appreciable amounts of olefins, such as HCFO-1233xf, in the HCFC-244bb feed material to the next step of the 2,3,3,3-tetrafluoropropene manufacturing process may be detrimental to the dehydrochlorination catalyst.
Previously, various methods of separating azeotropic mixtures of fluorocarbons have been suggested. European patent application EP 0 472 391, for example, suggests separating HFC-134a from a mixture containing hydrochlorofluorocarbons using an extraction agent such as trichloroethylene or perchloroethylene, among others. U.S. Pat. No. 5,211,817 also attempts a separation of fluorocarbons from azeotropic mixtures with HF by column distillation and withdrawing a vapor sidestream followed by introducing the sidestream into a rectifying column equipped with a condenser and operated at a high reflux ratio. These proposals, however, provide less than satisfactory solutions to the instant problem. Thus, there is a need for a new manufacturing process for the production of tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene.